The present invention relates to ion implantation of selected impurities or dopants into a substrate of semiconductive material without utilizing a mask, and more particularly, to form ion implanted pattern by selectively scanning focused ion beam on the surface of a processing substrate. It provides a high precision device, capable of achieving the critical accuracies required by large scale integration.
In the production of elemental devices such as transistor, in a large scale integration (LSI) circuit, it is generally necessary to introduce dopants or impurities into the crystal structure of a semiconductor material. These dopants must be introduced in a specific configuration and within very close tolerances. Introducing the impurities in the required patterns and achieving the desired resolution has proven to be a difficult problem. Impurities have been diffused through a mask which has been formed into a desired configuration by photolithographic processes on the substrate. However, this method allows the dopants to migrate laterally under the mask, thus adversely affects to resolution. The diffusion method also requires multi-step photolithographic processes which are time consuming and expensive. One solution to minimize the adverse resolution by the diffusion is to introduce impurity by means of ion implantation. With ion implantation, the dopant material is ionized and then accelerated into a target substrate through a mask which defines the desired pattern.
Ion implantation methods of the prior art have generally required the use of masks, and it is customary to use an ion beam to implant dopants through a mask placed either directly upon or spaced some distance from the target substrate. Ion implantation with the mask located directly on the substrate, though reducing the lateral migration associated with the diffusion, still is subject to the costs of the photolithographic process.
On the other hand, ion implantation through an apertured mask spaced from the substrate, eliminates the necessity of masking and etching step; however, new problems occurs in this approach. Because of the necessity of providing supports for the mask, certain configurations may not be obtained by use of masks spaced from the target substrate. For example, using a mask which is spaced from the substrate, it is impossible to implant dopants in an annular or other closed-loop pattern in a single step since the supports for the mask will cast a shade of itself on the substrate. The prior art has recognized the need for a method of ion implantation which avoids these difficulties and thus, maskless ion implantation system have been proposed.
To clarify the advantages of the present invention over the prior art, a prior process, as illustrated in FIG. 1 through FIG. 2, will be described briefly. FIG. 1A is a schematic plan view of the layout of a conventional gallium arsenide (GaAs) field effect transistor (FET). The FET has a gate electrode G, a source region S, a drain region d and channel region C. A distance from a source S to a drain d is a channel length. It has previously been proposed high performance devices typically having channel lengths of 3-6 microns.
As the scale of integration of semiconductor device increased, minimum dimension of patterns become less than a micron. In such high precision device, it is necessary to attain the critical accuracies of the device.
FIG. 1B is a schematic plan view illustrating a focused ion beam scanning. Ions are implanted into the substrate area S, by scanning the ion beam b with a pitch P, for example 0.05 to 0.1 .mu.m, on the substrate expose and step operation. It is reported that an ion implanted pattern has been formed by selectively scanning the focused ion beam at the beam diameters range from about 0.1-1.0 .mu.m without utilizing a mask.
FIG. 2 is a schematic block diagram illustrating a prior art apparatus for selectively implanting dopants or impurity ions into a substrate of semiconductor material without utilizing a mask. Generally, the apparatus comprises five main sections:
1. Ion source and focusing system PA1 2. High voltage source PA1 3. Low voltage source PA1 4. Stage for substrate and vacuum system PA1 5. Computer and peripherals
But, the detailed description of each elements are omitted because they are common in the art.
FIG. 2, illustrates general configuration of ion implantation system. Ions emitted from liquid metal ion (LMI) source 1 are controlled by an ion control electrode 2, beam alignments 4a, 4b, and 4c, and a blanking electrode 6. And filtered by E.times.B mass filter 7a which selects the ion and purity of the dopant by electric and magnetic cross field and mass separator slit 7b, finally the doping ions are focused onto a target 12 with approximately unitary magnification. Typically, the doping ions have been focused to spot diameters ranging from 0.1 to 3.0 .mu.m at about 50 keV with a constant current density of 0.5 (A-/cm.sup.2). In addition to focusing, the lens 3, 5, and 8 also accelerate or decelerate the ion beam. The final beam energy at the target 12 on the stage 11 can be varied from 40 to 200 keV. The ion source 1 is movable to align the ion beam with the electro-optical axis of the lens.
The deflector 9 is used to electrostatically scan the ion beam across the target 12 and for calibrating astigmatism. A 100 keV ion beam can be deflected over the entire scan field of 500 .mu.m retaining a 0.1 .mu.m diameter of focused beam. A central processing unit (CPU) 21 controls an ion beam system of the apparatus. The CPU 21 receives an input data which includes pattern data 22 and doping data 23 stored in magnetic tape, and data coming from probe 10 for ion current measuring. Then the CPU 21 generates control signals to the element of the apparatus to control the system, such as the lens system, a faraday cup 10, the stage 11, and a pattern generator 24.
The implantation dose D (ions/cm.sup.2) is given by: ##EQU1## where, I.sub.p is ion current (A), K is charge state of ions, S is an area of implanted region, and q is an electric charge unit (1.602.times.10.sup.-19 coulomb). Therefore, the implantation dose D (ions/cm.sup.2) is controlled by ion current Ip and ion implanting time T.
Generally, the ion beam is scanned step by step (not continuously) as shown in FIG. 1B. Area S for ion implanted region is given as EQU S=mP.multidot.nP=mnP.sup.2 ( 2)
where, S is a target area, m and n are respectively number of clock pulse for scanning to X and Y direction of the target area. If clock frequency is fc, the scanning time t to scan an area S can be obtained by the following equation: EQU t=mn/fc (3)
when charge state of ion beam k is one (K=1), an implantation dose D.sub.1 in the area S in one scanning is given by ##EQU2## It is clear from the equation (4), the implanted dose numbers D.sub.1 is proportional to the ion current Ip which varies proportionally with ion beam diameter and ion beam density, and inversely proportional to the clock frequency fc and squares of pitch (P.sup.2).
Generally, in order to obtain the desired dose D (ions/cm.sup.2), it is necessary N times repetition of scan the target. So, the dose is given as EQU D=N.multidot.D.sub.1 ( 5)
In the equations (4) and (5), the ion current Ip and the scanning number N must be controlled in order to obtain a desired dose D. But when ion current Ip is varied, the focus system must be adjusted in order to attain high resolution. So, it is very difficult to vary the ion current Ip in the prior art apparatus. On the other hand, the scanning of the ion beam is linked to the clock frequency and the clock frequency fc is fixed for prior art ion implantation apparatus. Therefore, the dose can be varied discretely by varying the number of scan N, and it is impossible to vary the dose D precisely.
In conventional ion implantation apparatus, the clock frequency fc has been determined by count down of a standard clock frequency. So the frequency can be varied discretely to 1/2.sup.n times of the standard clock frequency. For example, for the standard clock frequency of 10 MHz, the clock frequency can be only selected to 10.0, 5.0, and 2.5, (MHz) etc. Since the clock frequency is not controlled continuously, the dose can not be controlled continuously.